Silicone fluids are known additives for personal care products. Silicone fluids such as dimethicone and cyclomethicone provide hair feel and conditioning benefits in personal care compositions. Physical properties of silicone fluid polymers, including viscosity and solubility, are typically determined by molecular weight.
Unlike silicone fluids, silicone elastomers are cross-linked. The creation of cross-linkages between linear polymers, such as dimethicone, converts the linear polymer into a silicone elastomer. In contrast to silicone fluid polymers, the physical properties of elastomers are typically dependent on the number of cross-linkages, rather than molecular weight. The ability of silicone elastomers to swell makes them ideal thickeners for oil phases. The elastomers have a very smooth and soft feel when applied to skin or hair. They can also be used as delivery agents for fragrances, vitamins and other additives in cosmetic compositions.
Typical silicone elastomers are swollen in cyclic or low molecular weight silicone fluids. Elastomers have historically been created using two different processes, suspension or solvent polymerization. In the suspension process, an initiator and cross-linking agent are added to an emulsion of the pre-cursor silicone polymer and surfactant to begin the cross-linking. Heating of the suspension drives the reaction to completion and the small dispersed droplets of precursor polymer become spherical elastomer particles. The resulting elastomer may then be spray-dried and sold as a powder or used in the emulsified form which is typically less than five microns.
The powder must be swollen in a suitable low molecular weight solvent (typically <1000 cs) before use. In the solvent process, the pre-cursor silicone polymer is solubilized, then is cross-linked by catalyst addition and a cross-linking agent. Solvents for this process are generally cylcomethicones but may also include low viscosity dimethicones or organic solvents (typically <1000 cs). The resulting gel is then broken into small particles which may be sold as a paste.
In both processes, low molecular weight or cyclic silicones are necessary for swelling the particles or for the actual cross-linking process (New Developments in Silicone Elastomers for Skin Care, M. Starch.) In aqueous shampoo applications, quantities of low molecular weight or cyclic silicones results in disruption of surfactant packing and significant decrease in both lather amount and overall viscosity. Therefore, known silicone elastomers are not well-suited for many personal care compositions, especially those that rely on surfactant structuring to achieve viscosity targets.
Based on the foregoing, there is a need for a silicone elastomer which maintains the stability and performance (i.e. does not disrupt packaging, appearance, or lather) of personal compositions.